1. Field of the Invention
The present invention relates to an electronic still camera having image pickup elements with a plurality of color filters, and more particularly, to orthogonal transformation and coding used for image processing.
2. Related Background Art
An electronic still camera (also called a still video camera) stores image signals taken by solid state image pickup elements such as CCDs, in a storage unit such as a memory card. It is necessary to compress the image signal data to a suitable capacity, in view of a relative small capacity of the storage unit and a large amount of information pertaining to the images. As a method of data compressing, an orthogonal transformation and coding method is known. With this method, image data is divided into a plurality of blocks, and each pixel in a divided block is subject to orthogonal transformation and coding. The characteristic feature of this method is as follows. Namely, while considering the visual characteristic of human eyes, a larger number of bits are assigned to low frequency components of an image signal to finely quantize it, and a smaller number of bits are assigned to high frequency components of the image signal to coarsely quantize it. In this manner, redundant information within a block is suppressed to reduce the data amount of an image signal, without deteriorating the quality of an image.
As a method of improving compression coding factor in accordance with the layout of color filters mounted on an image pickup device, Japanese Patent Laid-open Publication No. 1-154696 discloses that color image data from color filters is subject to orthogonal transformation for each color.
The conventional method disclosed in the above-cited publication has the following disadvantages. In the case of the layout of color filters such as shown in FIG. 1, the phases (spatial positions) of pixels of R (red), G (green), and B (blue) are the same in the vertical and horizontal directions as shown in FIG. 2A to 2C, so that compression factor can be improved without deteriorating the image quality. However, in the case of the layout of color filters such as shown in FIG. 3, called a Bayer matrix, the phases of respective colors R, G, and B become as shown in FIGS. 4A to 4C. For orthogonal transformation for each color, particularly for G color, pixels with a shifted phase in each line as shown in FIG. 4A are subject to orthogonal transformation, resulting in less correlation between lines and hence deterioration of image quality and lowering of compression factor.